Rotoscoping is a motion picture process for removing an unwanted background from a photographic image. patent application Ser. No. 09/008,270 filed Jan. 16, 1998, describes a computer aided method for removing, from an image, the background surrounding a selected subject, including those elements of the background seen through transparent, semitransparent and translucent areas. (For brevity, the term semitransparent is intended to include the terms transparent and translucent.) The method also creates a matte (alpha channel) which permits later compositing of the subject over another background. The new background is visible, through semitransparent subject areas, to the extent the original background was visible. Removal of the background in semitransparent areas is especially important in motion pictures because moving subjects photographed at low shutter speeds always produce a semitransparent blurred edge.
Blurred edges are usually not a problem for still images used in graphic arts because high shutter speeds or strobe lights can eliminate unwanted movement blur. However, the referenced method is very useful for stripping or masking still images when portions of the subject are transparent, semitransparent of translucent.
The referenced method is inadequate for certain semitransparent portions of an image where good estimates of the subject and background colors could not be obtained, thus limiting its full potential. Extrapolation of these not very good estimates can therefore fail to correctly generate the required subject and background reference colors. The result is an inaccurate matte that does not match the level of, nor properly remove, the background in semitransparent areas of the subject. Every new subject, background, or camera angle, produces a difference photographic image. There can therefore exist an infinite number of images. If a new background removal method is to be accepted, it must perform well on more than a few selected images. The technology contained in the above described invention, when provided with good estimates of the subject and background colors, is sufficiently comprehensive to permit accurate extraction of a subject for almost every image.
The present invention provides improved estimates of the subject and background colors contributing to the color observed at a given pixel in transparent, semitransparent, or translucent subject areas. In this sense a shadow is a non reflective transparent subject having an inherent density greater than zero.